1. Field of the Invention
The present invention relates to the coloration of titanium and its alloys, which have been increasingly used as decorative and corrosion-resistant materials in recent years. In particular, this invention relates to a novel process for coloring titanium and its alloys in which the coloring is adjusted by controlling the supplied amount of current. The process is quite different from conventional coloring processes by anodic oxidation in which the coloring has been adjusted by means of voltage control.
2. Description of the Prior Art
Titanium has found growing applications, especially in structural materials of aerospace crafts or nuclear power plants or other chemical industrial materials, because of its advantages of lightness of weight, high specific strength and superior high corrosion-resistance. Further, in recent years, titanium has found new applications in building materials, for example, for roofs and curtain walls and other interior structural members. Particularly, in building materials, it is necessary to provide colors onto the surfaces of such building materials by anodic oxidation, etc., with a view to providing high levels of artistic effects. Many studies have been made on such coloring films.
The coloration of titanium has been heretofore achieved by employing interference colors which result from thin oxide films formed onto the surface of titanium metal by means of anodic oxidation using titanium as an anode in certain electrolytic solutions. The resulting interference color is changed to various color tones depending upon the thickness of the formed anodic oxide film. Further, since there is a direct correlation between the anodic oxide film and the applied voltage, delicate color control can be effected by controlling the applied voltage. Currently, practical processes utilize the foregoing characteristic aspects.
Among the above studies, the most practical anodic oxidation is carried out by applying a direct voltage to an electrolytic solution containing, for example, phosphoric acid, sulfuric acid or boric acid, using titanium as an anode and thereby forming an oxide film onto the surface of the titanium and growing the oxide film. In such a process, the thickness of the resultant oxide film is variable depending upon the applied voltage and the light interference also differs depending on the thickness of the oxide film. Consequently, various color tones are produced. For instance, when an electrolytic solution of phosphoric acid is employed, the anodic oxide film is colored in blue tones by applying a voltage of 25 volts and, with increasing the applied voltage, the anodic oxide film becomes thicker and the interference color by the surface film changes to various colors, for example, to yellow, to pink, to purple and to green. When a voltage of 120 volts is applied, the color turns into a reddish violet color. Therefore, color adjustment has been effected by voltage control and, when various color tones are desired, it is essential to use an electric power unit having a high withstand voltage. Generally, an electric power unit having a withstand voltage of the order of at least 150 volts is needed.
As set forth above about the prior art processes, an electric power unit with a high withstand voltage must be employed in order to form a variety of color tones on titanium. On the other hand, presently industrialized anodic oxidation processes, for example, an aluminum anodic oxidation process, employ an electric power source unit having a low withstand voltage ranging from 20 to 30 volts for the formation and growth of an anodic oxide film. Therefore, if the coloration of titanium becomes possible employing such an electric power unit with a low withstand voltage, the above electric power unit currently used would be also used for the coloration of titanium and a further expanded application could be expected.